1. Field of the Invention
The present invention relates to a radiation image reading apparatus for deriving image signals in such a manner that a slab-like shaped accelerated phosphorescence fluorescent material object, on which radiation images are accumulated and stored, is scanned with excitation light, and an accelerated phosphorescence fluorescent light, which is emitted from the accelerated phosphorescence fluorescent material object, is received.
2. Description of the Related Art
Radiation images such as x-ray images have been often used for a medical diagnosis. For example, in case of x-ray images, an x-ray transmitted through a subject is applied to a fluorescent material layer (fluorescent screen) so as to be converted into a visible light. The visible light is applied to a silver halide film to form a latent image. The x-ray image is obtained by developing this silver halide film. The x-ray images thus obtained are used for medical diagnosis. Recently, there has been adopted a system for obtaining a high quality of reproduced image wherein the x-ray image formed on a silver halide film as described above is read photoelectrically by a so-called film digitizer to derive image signals and these image signals are subjected to an image processing to improve various image characteristics which determine the image qualities, such as definition, dynamic range, graininess and the like.
Instead of the above-described system using silver halide films, a system using an accumulative fluorescent material (accelerated phosphorescence fluorescent material) has begun to be used. The system using the accelerated phosphorescence fluorescent material is a system in which the accelerated phosphorescence fluorescent material in a shape of sheet or panel is irradiated with an x-ray transmitted through the subject so that x-ray images are accumulated and stored in the accelerated phosphorescence fluorescent material, then the accelerated phosphorescence fluorescent material is scanned with an excitation light to emit an accelerated phosphorescence fluorescent light carrying the x-ray images stored therein, and the accelerated phosphorescence fluorescent light is photoelectrically read to derive image signals and the image signals thus derived are subjected to an image processing to obtain reproduced images. The basic method of this system is disclosed in U.S. Pat. No. 3,859,527. Here, the term "accelerated phosphorescence fluorescent material" refers to a fluorescent material which accumulates therein part of energy of radiation for a while or a long period of time when it is irradiated with radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays or the like, and emits the accumulated energy as an accelerated phosphorescence fluorescent light when it is irradiated with an excitation light such as infrared radiation, visible light, ultraviolet radiation or the like during such an accumulation. In this case, a type of radiation the energy of which is prone to be accumulated, a wavelength of an excitation light which facilitates the emission of an accelerated phosphorescence fluorescent light and a wavelength of an accelerated phosphorescence fluorescent light to be emitted differ depending on the type of a fluorescent material to be used.
According to a system using such an accelerated phosphorescence fluorescent material, it is recognized that an energy of radiation applied to the accelerated phosphorescence fluorescent material is proportional to a light quantity of the accelerated phosphorescence fluorescent light emitted by irradiation of the excitation light over the wide range of energy and the ratio of proportion can be changed in accordance with the light quantity of the excitation light. Thus, it is possible to obtain a radiation image which will not be affected by variations of exposure of radiation. In case of a system in which X-ray images of a human body is obtained, it is possible to reduce the exposure on the human body in the X-ray radiography.
A system using the accelerated phosphorescence fluorescent material as mentioned above involves the following four steps throughout the radiography of X-ray images. First, X-ray image information of a subject is accumulated and stored in the accelerated phosphorescence fluorescent material. This step is referred to as "storage". Next, the accelerated phosphorescence fluorescent material, which is subjected to the "storage" process, is scanned with the excitation light such as a laser beam so as to emanate the accelerated phosphorescence fluorescent light. The accelerated phosphorescence fluorescent light is condensed and received with a photoelectric converter, thereby deriving an electric signal which is proportional to the intensity of the accumulated radiation. This step is referred to as "reading". Thereafter, the electric signal thus derived is subjected to an image processing in accordance with necessity and a visible radiation image is obtained by means of printing on a silver film or displaying on a CRT. This step is referred to as "display". Even after the "reading", the radiation energy still remains in the accelerated phosphorescence fluorescent material. The residual energy is removed in such a manner that after the reading the accelerated phosphorescence fluorescent material is irradiated with a relatively strong excitation beam to emanate the accelerated phosphorescence fluorescent light. This step is referred to as "erasing". In case of the radiography of a plurality of sheets of X-ray image, the above-mentioned four steps or three steps of "storage", "reading" and "erasing" are repeated but with "display" in the final step.
FIG. 18 is an illustration showing the basic arrangement of constituents of a radiation image reading apparatus using an accelerated phosphorescence fluorescent material. FIG. 19 is a sectional view of a substage condenser which is used in the radiation image reading apparatus as shown in FIG. 18.
First, a slab-like shaped accelerated phosphorescence fluorescent material object 1, which is accommodated in a cover member (not illustrated) for the apparatus, is irradiated with x-rays emitted from an x-ray source 2 passing through a subject 3, so that an x-ray image of the subject 3 is accumulated and stored in the accelerated phosphorescence fluorescent material object 1. In reading, a laser beam source 4 emits a laser beam 4a as an excitation beam. The emitted laser beam 4a is reflected by a mirror 5 and then deflected by a polygonal mirror 6. Thereafter, the deflected beam is corrected by an f-.theta. lens 7 so that a laser spot runs straight in uniform motion on the accelerated phosphorescence fluorescent material object 1, and then applied via a first aperture 8a and a second aperture 8b of a substage condenser 8 to the accelerated phosphorescence fluorescent material object 1. The accelerated phosphorescence fluorescent material object 1 is scanned with the beam thus generated in a main scanning direction. An accelerated phosphorescence fluorescent light, which is emanated through scanning once in the main scanning direction with the laser beam 4a (excitation beam), is incident upon the substage condenser 8 through the second aperture 8b thereof. The substage condenser 8 is provided with a cavity 8c inside thereof. The accelerated phosphorescence fluorescent light incident upon the substage condenser 8 through the second aperture 8b travels in the cavity 8c toward a photomultiplier 9, while repeating the reflection from the inner wall of the cavity 8c.
Between the substage condenser 8 and the photomultiplier 9, there is provided an optical filter (not illustrated) which inhibits light having a wavelength of the excitation laser beam from being transmitted and permits only the accelerated phosphorescence fluorescent light to be transmitted. The accelerated phosphorescence fluorescent light selectively derived through such an optical filter is converted into an electric signal by the photomultiplier 9, and the electric signal thus obtained is amplified in signal level by an amplifier 10 to a level which is optimum for an A/D conversion. The electric signal thus amplified is converted into a digital signal by an A/D converter 11 so as to derive a line of time-sequence data as to an image corresponding to one scanning by the excitation beam (laser beam).
A single unit, which is referred to as a reading unit, is constituted of: a scanning system comprising the laser beam source 4, the mirror 5, the polygonal mirror 6, the f-.theta. lens 7 and the like; a light receiving system comprising the substage condenser 8, the photomultiplier 9 and the like; and an erasing light source 12 for emanating an erasing light. The reading unit is arranged to be freely movable in both the directions of arrows X and Y as shown in FIG. 18. In reading, first, the reading unit is located at the topmost position in the direction of arrow Y, and then the above-mentioned main scanning is repeated while the reading unit is moved (sub-scanning) in the direction of arrow X. In this manner, the image accumulated and stored in the accelerated phosphorescence fluorescent material object 1 is read in its entirety and be converted into image data.
When the reading unit reaches the lowest position in the direction of arrow X through the reading mentioned above, the laser beam source 4 turns off, whereas the erasing light source 12 turns on and the reading unit moves in the direction of arrow Y. During a movement of the reading unit, the erasing light emanated from the erasing light source 12 is applied to the accelerated phosphorescence fluorescent material object 1 so that the energy (residual image) remaining in the accelerated phosphorescence fluorescent material object 1 is erased.
FIGS. 20 and 21 are each a view showing by way of example a substage condenser which is different in type from the substage condenser 8 shown in FIG. 19.
Hitherto, as a substage condenser for guiding the accelerated phosphorescence fluorescent light into a photoelectric converter, there are further known, in type other than that having the cavity shown in FIG. 19, a so-called bundle type of condenser in which a number of plastic fibers or glass fibers are bundled, as shown in FIG. 20, and a so-called light guide sheet type of condenser in which a plastic plate such as an acrylic sheet is bent. The substage condensers shown in FIGS. 20 and 21 have each a configuration adapted to guide the accelerated phosphorescence fluorescent light, which is emanated on the scanning line in straight, into a photoelectric converter having a circular photoelectric surface.
The radiation image reading apparatus mentioned above as the related art has been associated with the following drawbacks with respect to the reading.
It is known that vibrations of a photomultiplier involve the superposition of noises upon an electric signal outputted from the photomultiplier. A mechanical movement of the photomultiplier as shown in FIG. 18 involves such a problem that minute vibrations involved in the movement are transmitted to the photomultiplier whereby noises are superposed upon a reproduced image.
A traveling path for the reading unit forms a dead space which inhibits other parts from being disposed. Thus, for example, a power supply, a circuit substrate and the like, which are needed to be arranged in the apparatus, are obliged to be disposed in a place other than the dead space. This causes an apparatus of a larger size to be needed.
In order to prevent the superposition of noises due to vibrations of the photomultiplier, it is considered that the photoelectric converter is fixed, whereas the accelerated phosphorescence fluorescent material object is moved. However, since the accelerated phosphorescence fluorescent material object has a slab-like shaped wide area, it will be needed to provide a larger dead space which enables the accelerated phosphorescence fluorescent material object to be moved.
The radiation image reading apparatus mentioned above as the related art has been associated with the following drawbacks also with respect to the erasing.
In order to perform the erasing efficiently with a low electric power and in a short time, it is desired that a wavelength band of a relative spectral energy distribution of the erasing light source is within a wavelength band of an accelerated phosphorescence excitation spectrum, and in addition a lamp efficiency (luminous flux per unit dissipation power) of the erasing light source is high. By the way, when the accelerated phosphorescence fluorescent material object Ba Br.sub.2 :Eu is used for a latent image storage, the wavelength band of an accelerated phosphorescence excitation spectrum is approximately coincident with the visible light region, whereas the wavelength band of a relative spectral energy distribution of the tungsten halogen lamp used usually as the erasing light source overlaps with the visible light region but in major part spreads over the infrared region. In other words, an amount of energy for use in the erasing, among the energy supplied to the tungsten halogen lamp, is only a small amount converted as the visible light, and the remaining major part of the energy is emitted as heat.
When it is intended to perform the erasing in a short time, there is a need to apply the strong visible light to the accelerated phosphorescence fluorescent material object. Consequently, in order to obtain the strong visible light with the use of the tungsten halogen lamp having the characteristics as mentioned above, it is necessary to use a tungsten halogen lamp which is of very high dissipation power in the order of several kilo-watts, and disposes the lamp near the accelerated phosphorescence fluorescent material object and move it along the accelerated phosphorescence fluorescent material object.
A high dissipation power of tungsten halogen lamp emanates intense heat. This heat diffuses within a cover member of the radiation image reading apparatus to increase the temperature within the cover member of the apparatus. Increased temperature causes the accelerated phosphorescence fluorescent light emanated on the accelerated phosphorescence fluorescent material object to vary in level. This affects a signal level of the X-ray image. Further, this affects thermally various electric circuits which are incorporated in the cover member. This will cause a malfunction. In view of the foregoing, in order to prevent a thermal diffusion from the tungsten halogen lamp, generally, there is provided an ventilating arrangement for exchanging warm air surrounding the tungsten halogen lamp and cool air outside the cover member. Thus, a large occupation space is needed. This requires a large sized apparatus.
On the other hand, there is known a system in which an erasing light source is fixed and an accelerated phosphorescence fluorescent material object is irradiated with erasing light in its entirety all at once. However, according to this system, there is a need to provide an optical path from the erasing light source to the accelerated phosphorescence fluorescent material object. In other words, the optical path forms a dead space. This requires a large sized apparatus, since the units, which are to be incorporated in the cover member of the apparatus, cannot be disposed on the optical path and there is a need to install the units outside the optical path.
In view of the foregoing, it is an object of the present invention to provide a radiation image reading apparatus capable of obtaining image data while reducing noises.
It is another object of the present invention to provide a radiation image reading apparatus which contributes to miniaturization of the apparatus.